This invention relates to the treatment of exhaust gas emitted from combustion systems such as waste incinerators, boilers and the like, and more particularly to an improved method and device for reducing the temperature of exhaust gas to substantially reduce the size of the device, to prevent damage to a gas cooling chamber or an exhaust gas duct caused by sprayed water, to eliminate operational difficulties caused by the deposit of dust, and also to remove acidic gas contained in the exhaust gas.
Conventionally, exhaust gas emitted from combustion systems such as waste incinerators, boilers and the like is diffused into the atmosphere after being purified by a gas purification device.
In some cases of purification treatment, it has been found necessary that the temperature of the exhaust gas be reduced to an appropriate temperature, for example, approximately 120-250xc2x0 C. depending on the gas purification device used. Conventionally, in such cases, gas purification devices spray water into the exhaust gas and utilise its heat capacity and latent heat of evaporation to reduce the temperature of the exhaust gas.
Referring to FIG. 9 and FIG. 10, examples are shown of a conventional device to reduce the temperature of exhaust gas, wherein 21 is a gas cooling chamber, 21a is an exhaust gas inlet, 21b is an exhaust gas outlet, 21c is an ash outlet, 22 is a temperature reduction water tank, 23 is a pressure pump, 24 is a temperature reduction water nozzle, 25 is a temperature control device, 25a is a temperature detector, 26 is a temperature reduction water volume control valve, 27 is a injection pump, 28 is an air compressor, 29 is a compressed air tank, 30 is a mixer, Gh is high temperature exhaust gas, Gl is low temperature exhaust gas and C is ash.
With reference to the device for reducing the temperature of exhaust gas in FIG. 9, high pressure water from the temperature reduction water tank 22, pressurized by the pressure pump 23, is sprayed into the gas cooling chamber 21 through the temperature reduction water nozzle 24 provided in the vicinity of the exhaust gas inlet 21a. The temperature of sprayed water rises in contact with high temperature exhaust gas Gh, and is vaporised to become steam when it reaches its boiling point.
On the other hand, high temperature exhaust gas Gh in the gas cooling chamber 21 is cooled by the heat capacity of the sprayed water, latent heat of evaporation and the heat capacity of the steam, thus lowering the temperature to a prescribed temperature so as to be led out of the exhaust gas outlet.
The volume of water to be sprayed into the gas cooling chamber 21 is controlled by adjusting the opening of the temperature reduction water volume control valve 26 through the temperature control device 25 in response to temperature detecting signals from the temperature detector 25a. The temperature of low temperature exhaust gas G led out of the exhaust gas outlet 21b is maintained at a desired temperature by controlling water volume to be sprayed into the gas cooling chamber 21 by means of controlling the volume of water returned to the temperature reduction water tank 22.
With reference to the device for reducing the temperature of exhaust gas shown in FIG. 10, water sent from the temperature reduction water tank 22 by means of the injection pump 27 and high pressured air sent from the compressed air tank 29 are mixed for atomisation in the mixer 30. Atomised water is sprayed into the gas cooling chamber 21 from the mixer 30, through the temperature reduction water nozzle provided in the vicinity of the exhaust gas inlet 21a. 
Features such as (1) that the temperature of sprayed water rises in contact with high pressure exhaust gas Gh and is vaporised to become vapour steam when it has reached its boiling point, (2) that high temperature exhaust gas Gh in the gas cooling chamber 21 is cooled by the heat capacity of the sprayed water, latent heat of evaporation and heat capacity of the steam vapour, (3) that the water volume to be sprayed into the gas cooling chamber 21 is controlled by adjusting the opening of the temperature reduction water volume control valve 26 through the temperature control device 25, and (4) that the temperature of low temperature exhaust gas G is maintained at a desired temperature by controlling water volume to be sprayed, are all precisely the same as those features in FIG. 9. The previous devices for the temperature reduction of exhaust gas shown in FIGS. 9 and 10 are capable of reducing the temperature of high temperature exhaust gas Gh to a desired temperature by utilising low cost water, thus achieving excellent and practical effects.
There remain, however, a number of difficulties related to the aforementioned prior devices for temperature reduction of exhaust gas, of which, major difficulties include (a) that refractories are damaged by the downflow of water droplets when they hit the wall surface of the gas cooling chaser directly, (b) that stable operation of the gas cooling chamber is impaired by dust adhered to and deposited on the wall surface, and (c) that it is difficult to provide a device for temperature reduction of exhaust gas having a small size since the gas cooling chamber remains large in size.
In the event of a single fluid method wherein only water is utilized, as shown in FIG. 9, difficulties remain in making the atomized temperature reduction water have particles of micro-sized diameters, even by increasing the pressure of the water or making improvements in the water nozzle 24. With this method, the diameters of atomised particles of temperature reduction water normally stay coarse, having diameters around 70-200 xcexcm, which makes it difficult for the atomised temperature reduction water to be thoroughly vaporised within a limited space, thus causing damage to the refractory when water droplets hit the wall surface of the gas cooling chamber directly.
Even when damage to the refractory is avoided, there is a possibility that dust would adhere to and deposit at the surface of the refractory that is wet with water droplets, that deposits adhered to the surface of the refractory would gradually grow, and that the passage resistance of exhaust gas in the gas cooling chamber would increase and fluctuate considerably, thus making the smooth operation of the gas cooling chamber difficult.
With a double fluid method shown in FIG. 10, wherein water and compressed air are employed, the diameters of atomised particles of temperature reduction water normally become around 30-100 xcexcm thus reducing the frequency of problems in comparison with the single fluid method.
However, this double fluid method is not ideal from the viewpoint of cost because of the high initial and running costs of compressed air equipment.
Furthermore, the time required before the aforementioned atomised cooling water reaches its boiling point and evaporates thoroughly is considerably long. This means that it becomes necessary for the retention time of exhaust gas in a gas cooling chamber to be sufficiently long, thus requiring a gas cooling chamber of a large capacity.
For example, in the case of an industrial waste incinerator with a capacity to handle incineration disposal of industrial waste of approximately 300 T/D (tons per day), assuming high temperature exhaust gas Gh with an exhaust gas volume of 90,000 Nm3/H (flow rate of gas with volume converted to normal or standard volume) and an inlet exhaust gas temperature of 240xc2x0 C. is converted to low temperature exhaust gas G with an inlet gas temperature of 180xc2x0 C., a gas cooling chamber of an internal diameter of approximately 4,800 mm and the height of approximately 9,000 mm is required with a device for temperature reduction of exhaust gas by means of a single fluid method as shown in FIG. 9. Thus, the total height of the device for temperature reduction of exhaust gas including an exhaust gas inlet 21a, an exhaust gas outlet 21b and an ash outlet 21c would be approximately 180,000 mm.
When designing previous devices for temperature reduction of exhaust gas, the heat load of the gas cooling chamber is normally chosen to have a value of 5,000-10,000 kcal/m3xc2x7H (heat value taken away from exhaust gas per unit volume and unit time of a gas cooling chamber in units of kilocalories per meter cubed per hour). For example, the heat load of the gas cooling chamber is chosen to be 7,000 kcal/m3xc2x7H.
The present invention is concerned with solving the aforementioned problems with the prior devices for temperature reduction of exhaust gas, namely, (a) that, due to coarse particle diameters of atomised temperature reduction water, water droplets directly hitting the wall surface of the gas cooling chamber cause damage to the refractory, and, due to dust adhered to and deposited on the wall surface, the smooth operation of the gas cooling chamber is disturbed with the single fluid method, (b) that pressurized air equipment is required, thus increasing both initial and running costs with the double fluid method, and (c) that it becomes difficult to make the gas cooling chamber significantly smaller in size due to the time consumed before atomised water particles evaporate. Accordingly, it is an object of the present invention to provide a method and device for effective and economical temperature reduction of exhaust gas having an exceedingly small size by means of reducing the diameter of the atomised water particles.
The inventor of the present invention has acquired knowledge through designing, manufacturing and experimenting with numerous devices for temperature reduction of exhaust gas. For example, the inventor has found that with a device for temperature reduction of exhaust gas using the single fluid method wherein only water is employed, it is difficult to reduce the particle diameter of atomised temperature reduction water smaller than approximately 100 xcexcm just by providing an improved temperature reduction water nozzle or raising the pressure of temperature reduction water, as long as a temperature reduction water nozzle is employed for atomisation. Accordingly, it is difficult to provide a gas cooling chamber having a remarkably reduced volume.
The inventor of the present invention, therefore, has departed from the conventional idea of the design of the prior art type of device for temperature reduction of exhaust gas wherein water of a normal temperature of approximately 20-30xc2x0 C. is employed as temperature reduction water, and heat capacity of water and latent heat of evaporation are effectively utilized, and has come to the idea of a process wherein air-liquid of pressurized water at the boiling point of water under atmospheric pressure or pressurized thermal water partly containing steam is atomised and injected through a conventional temperature reduction nozzle.
When pressurized water of a temperature higher than the boiling point of water under atmospheric pressure is used, the heat value equivalent of the heat capacity of the water to be utilized for cooling exhaust gas is reduced compared with the prior single fluid method, thus resulting in a slight increase in the water volume required.
However, when said pressurized thermal water is sprayed through a temperature reduction water nozzle into a gas cooling chamber, there occurs so-called boiling under reduced pressure in the vicinity of the outlet of temperature reduction water nozzle, and particle diameters of atomised water become micro-sized in a range of approximately 3 xcexcm-50 xcexcm, and the water rapidly evaporates within a short period of time in a gas cooling chamber, thus enabling improvement of the cooling effect of exhaust gas and allowing the gas cooling chamber to be smaller in size.
The present invention has come into existence based upon the results of numerous experiments related to temperature reduction of exhaust gas based on the aforementioned ideas which go against the conventional technical common sense or practices carried out by others.
In a first embodiment according to the present invention, the present invention relates to spraying pressurized thermal water with a temperature higher than the boiling point of water under atmospheric pressure as temperature reduction water into exhaust gas.
In a second embodiment according to the present invention, the present invention relates to spraying pressurized thermal water with a temperature higher than the boiling point of water under atmospheric pressure as temperature reduction water into a gas cooling chamber or an exhaust gas duct.
In a third embodiment according to the present invention, thermal water taken out of a deaerator or continuous blow water of a boiler is utilized as part of the pressurized thermal water.
In a fourth embodiment according to the present invention, pressurized thermal water partly containing steam is used as temperature reduction water.
In a fifth embodiment according to the present invention, pressurized thermal water containing an alkaline solution is used as temperature reduction water.
In a sixth embodiment according to the present invention, heated alkaline solution is mixed into the thermal water.
In a seventh embodiment according to the present invention, alkaline solution is used as an alkaline aqueous solution or alkaline slurry solution.
In an eighth embodiment according to the present invention, an alkaline aqueous solution containing sodium hydroxide (caustic soda), or an alkaline slurry solution containing calcium hydroxide (slaked lime) is used.
In a ninth embodiment according to the present invention, the present invention comprises a gas cooling chamber equipped with a gas inlet, a gas outlet and an ash outlet, a thermal water tank for storing pressurized thermal water with a temperature higher than the boiling point of water under atmospheric pressure, a temperature reduction water nozzle to spray thermal water from the thermal water tank into the gas cooling chamber, a temperature reduction water volume control valve to adjust the volume of thermal water to be supplied to the temperature reduction water nozzle, a temperature detector for low temperature exhaust gas flowing from the gas outlet, and a temperature control device with an opening and closing mechanism for controlling the temperature reduction water volume control valve in response to detecting signals from the aforementioned temperature detector.
In a tenth embodiment according to the present invention, the present invention comprises an exhaust gas duct through which exhaust gas flows, a thermal water tank for storing pressurized thermal water with a temperature higher than the boiling point of water under atmospheric pressure, a temperature reduction water nozzle to spray thermal water from the thermal water tank into the exhaust gas duct, a temperature reduction water volume control valve to adjust the volume of thermal water supplied to the temperature reduction water nozzle, a temperature detector for low temperature exhaust gas flowing from the exhaust gas duct, and a temperature control device with an opening and closing mechanism for controlling the temperature reduction water volume control valve in response to detecting signals from the aforementioned temperature detector.
In an eleventh embodiment according to the present invention, thermal water is supplied to the temperature reduction water nozzle by means of an internal pressure of the thermal water tank.
In a twelfth embodiment according to the present invention, the basic constitution of the present invention comprises a gas cooling chamber equipped with a gas inlet, a gas outlet and an ash outlet, a thermal water tank for storing pressurized thermal water with a temperature higher than the boiling point of water under atmospheric pressure, an alkaline solution tank for storing alkaline solution, a mixer for mixing thermal water from the thermal water tank and alkaline solution from the alkaline solution tank, a temperature reduction water nozzle for spraying thermal water containing alkaline solution from the aforementioned mixer into the gas cooling chamber, a temperature reduction water volume control valve for adjusting the flow volume of thermal water containing alkaline solution to be supplied to the temperature reduction water nozzle, an alkaline solution volume control valve for adjusting the flow volume of alkaline solution to be supplied to the aforementioned mixer, a temperature detector for low temperature exhaust gas flowing from the gas outlet, an acid gas concentration detector for the aforementioned low temperature exhaust gas, a temperature control device with an opening and closing mechanism for controlling the temperature reduction water volume control valve by means of detecting signals from the aforementioned temperature detector, and an acid gas concentration control device with an opening and closing mechanism for controlling the alkaline solution volume control valve by means of detecting signals from the aforementioned acid gas concentration detector.
In a thirteenth embodiment according to the present invention, an alkaline solution heater for heating alkaline solution is installed in the alkaline solution inlet side of the mixer.
In a fourteenth embodiment according to the present invention, an alkaline solution tank is used for the alkaline solution tank for storing alkaline aqueous solution or alkaline slurry solution.
In a fifteenth embodiment according to the present invention, the present invention comprises an exhaust gas duct through which exhaust gas flows, a thermal water tank for storing pressurized thermal water with the temperature higher than a boiling point of water under atmospheric pressure, an alkaline solution tank for storing alkaline solution, a mixer for mixing thermal water from the thermal water tank and alkaline solution from the alkaline solution tank, a temperature reduction water nozzle for spraying thermal water containing alkaline solution from the aforementioned mixer into the exhaust gas duct, a temperature reduction water volume control valve for adjusting the flow volume of thermal water containing alkaline solution to be supplied to the temperature reduction water nozzle, an alkaline solution volume control valve for adjusting the flow volume of alkaline solution to be supplied to the aforementioned mixer, a temperature detector for low temperature exhaust gas flowed from the outlet of the exhaust gas duct, an acid gas concentration detector for the aforementioned low temperature exhaust gas, a temperature control device with an opening and closing mechanism for controlling the temperature reduction water volume in response to detecting signals from the aforementioned temperature detector, and an acid gas concentration control device with an opening and closing mechanism for controlling the alkaline solution volume control valve in response to detecting signals from the aforementioned acid gas concentration detector.
In a sixteenth embodiment according to the present invention, an alkaline solution heater for heating alkaline solution is installed in the alkaline solution inlet side of the mixer.
In a seventeenth embodiment according to the present invention, an alkaline solution tank is provided for storing alkaline aqueous solution or alkaline slurry solution.
Any combustion system such as a waste incinerator, a boiler and the like can be the emission source of the aforementioned high temperature exhaust gas, and the present invention is applicable to temperature reduction of all kinds of exhaust gas from combustion.
The temperature of high temperature exhaust gas Gh supplied to a gas cooling chamber can be fixed at the temperature of 150xc2x0 C.-1000xc2x0 C., and the temperature of low temperature exhaust gas G supplied from the gas cooling chamber can be fixed at a temperature higher than about 100xc2x0 C. For example, when the present invention is applied to primary cooling of exhaust gas, the temperature of high temperature exhaust gas Gh and the temperature of low temperature exhaust gas G can be fixed at approximately 900xc2x0 C.-1000xc2x0 C. and 150xc2x0 C.-250xc2x0 C., respectively. When the present invention is applied to secondary cooling of exhaust gas, the temperature of high temperature exhaust gas Gh and the temperature of low temperature exhaust gas G can be fixed at approximately 200xc2x0 C.-400xc2x0 C. and 120xc2x0 C.-250xc2x0 C. respectively.
The aforementioned gas cooling chamber can be formed in either a vertical shape or horizontal shape, and its cross section can be of any suitable shape, such as, for example, a circle, an ellipse or a square.
Similarly, the form of the aforementioned exhaust gas duct can be either of long width or short width, and its cross section can be of any suitable shape, such as, for example, a circle, an ellipse or a square.
The aforementioned pressurized thermal water Wt is water maintained at a temperature higher than the boiling point of water (100xc2x0 C.) under atmospheric pressure, or so-called water of high pressure and high temperature. The pressure of the pressurized thermal water Wt can be chosen at values of approximately 1 kg/cm2G-100 kg/cm2G. However, taking the pressure resistance of a thermal tank 2 into consideration, it is desirable that it is chosen somewhere between 3-10 kg/cm2G.
Pressurized thermal water Wt can partly contain steam. However, less steam content is desirable.
As a heat source for producing thermal water, water vapour from a waste heat boiler can be utilized if the combustion system, an incinerator, for example, is equipped with a waste heat boiler, and part of the vaporised steam can be utilized when the combustion system is a boiler.
With an incinerator not equipped with a waste heat boiler, either an exhaust heat exchanger is installed to utilise water vapour from the exchanger, or an independent steam or electric boiler of a small capacity can be installed.
When a deaerator is attached to the waste heat boiler of an incinerator or a boiler, thermal water produced in the deaerator can be used as temperature reduction water as it is. In this case, a device for temperature reduction can be constituted inexpensively because all that is needed for supplying thermal water Wt is installation of a pipe from the deaerator.
Furthermore, if a boiler or an incinerator equipped with waste heat boiler is used as a combustion system, continuous blow water from the boiler can be utilized as part of thermal water to be used as temperature reduction water. Most boiler equipment is designed so that part of boiler water (thermal water) is discharged outside to stop the rise of concentration of corrosion inhibitor and the like in the boiler water, so that a stable function of the corrosion inhibitor is performed. Boiler water discharged outside is normally found to be alkaline water of pH 8.5-11.8, having dechlorination or desulfurisation effects. Therefore, when chemicals are used for dechlorination or desulfurisation of exhaust gas, the amount of chemicals can be reduced.
If the gas cooling chamber is of a vertical type, it is desired that the temperature reduction water nozzle for atomising pressurized thermal water be installed at an upper part and in the vicinity of a gas inlet for high temperature exhaust gas Gh. The installation position of the temperature reduction water nozzle can be chosen as desired depending on the type of gas cooling chamber and the number of temperature reduction water nozzles to be installed. The same feature can be extended to the case wherein pressurized thermal water is injected inside an exhaust gas duct.
Any kind of suitable construction of nozzle such as water spray nozzles, for example, conventional screw type or collision type nozzles can be utilized.
Further, the number of temperature reduction water nozzles can be chosen as desired depending on such factors as the shape of the gas cooling chamber or exhaust gas duct, the number of ejection apertures installed at a nozzle, and the required ejection volume of thermal water, etc. For example, with a device for temperature reduction of exhaust gas for a prior art industrial waste incinerator (having a volume of incineration of 300 ton/D, a volume of exhaust gas of 90,000 Nm3/H, secondary cooling of exhaust gas (high temperature exhaust gas Gh at 240xc2x0 C. and low temperature exhaust gas G at 180xc2x0 C.), thermal water (saturated water of temperature 142.9xc2x0 C. and pressure 3 kg/m2 G), a temperature reduction water nozzle having three ejection apertures is installed at the upper part of the gas cooling chamber as described hereafter.
According to the present invention, the temperature and pressure of thermal water sprayed through a temperature reduction water nozzle becomes considerably higher than the boiling point of water under atmospheric pressure (100xc2x0 C.). Abrupt boiling under reduced pressure in the vicinity of an outlet of the nozzle ejection mouth produces micro-sized particles which instantly evaporate to water vapour after spraying, thus causing no direct impact of the wall surface of the gas cooling chamber liquid water droplets.
The results of the present invention, thus enable the volume of the gas cooling chamber to be small, reducing installation costs and space. For example, with a device for temperature reduction of exhaust gas equipped on the outlet side of a waste gas boiler of the aforementioned prior art industrial waste incinerator, when the temperature of high temperature exhaust gas is reduced from 240xc2x0 C. to 180xc2x0 C., it is found the heat load of the gas cooling chamber can achieve 50,000-150,000 kcal/m3xc2x7H.
Namely, compared with the heat load (5,000-10,000 kcal/m3xc2x7H) of a gas cooling chamber in a prior art device for temperature reduction of exhaust gas, the device for temperature reduction of exhaust gas in the present invention can be chosen at the range of 50,000-150,000 kcal/m3xc2x7H, thus enabling the volume of the gas cooling chamber to be reduced from 1/5-1/15.
Furthermore, the present invention allows the device to be constructed so that thermal water is directly sprayed into a high temperature exhaust gas duct by inserting a temperature reduction water nozzle, without providing a gas cooling chamber.
When thermal water is used for temperature reduction water, the volume of water sprayed increases slightly comparing with the case where low temperature water is used, as conventionally practised, due to the reason that the cooling capacity, which corresponds to the heat capacity, is lowered. For example, when the temperature of temperature reduction water with the conventional device for temperature reduction of exhaust gas is kept at 20xc2x0 C. and the temperature of thermal water with the device for temperature reduction of exhaust gas according to the present invention at 142.9xc2x0 C. (saturated water of pressure 3 kg/cm2G), it is found that approximately 1.2 times the volume of thermal water is required. This increase, however, does not require a larger size pipe for the thermal water line, thus not requiring much extra installation cost.
Thermal water mixed with alkaline solution used as temperature reduction water is ejected into exhaust gas through a temperature reduction water nozzle to remove hydrogen chloride (HCl) or sulphur oxide (SO2) contained in the exhaust gas.
The aforementioned alkaline solution can be in the form of either an alkaline aqueous solution or an alkaline slurry solution.
The temperature of the alkaline solution mixed in the thermal water is not necessarily required to be raised by heating when the temperature of temperature reduction water mixed with said alkaline solution is higher than the boiling point of water under atmospheric pressure. It is, however, desirable that, when the temperature of temperature reduction water becomes lower than the boiling point of water under atmospheric pressure by mixing with the alkaline solution, that the temperature reduction water be heated to a required temperature before the alkaline solution is mixed into the thermal water.
Any kind of alkaline agent in the aforementioned alkaline solution can be used. However, when it is used in the form of alkaline aqueous solution, sodium hydroxide (caustic soda, NaOH) or magnesium hydroxide (Mg(OH)2) are preferred.
When an alkaline slurry solution is used, sodium hydroxide (slaked lime, Ca (OH)2), quick lime (CaO), calcium carbonate (CaCO3), and sodium carbonate (Na2CO3) are preferred.
The total amount of alkaline agent in the alkaline solution to be mixed into the aforementioned thermal water is appropriately adjusted depending on the type and quantity of acidic gas in the exhaust gas and the temperature of the exhaust gas. Normally, alkaline agents of an equivalent ratio of 0.8-1.5 are mixed into the thermal water.
Further objects, features and advantages of the present invention will become apparent from the Detailed Description of Preferred Embodiments which follows, when considered together with the attached Drawings.